KR20140047960A - Ultra high strength cold rolled steel sheet having excellent weldability and bendability and method for manufacturinf the same - Google Patents

Abstract

The present invention relates to an ultra-high strength cold rolled steel sheet having excellent weldability and bendability and a method for manufacturing the ultra-high strength cold rolled steel sheet and, more specifically, an ultra-high strength cold rolled steel sheet having more than 980 Mpa of tensile strength enough to be used to manufacture cars and a method for manufacturing the ultra-high strength cold rolled steel sheet. An embodiment of the present invention provides the ultra-high strength cold rolled steel sheet which is composed of, in wt%: C: 0.08-0.105; Si: 0.05-0.6; Mn: 2.5-2.9; P: 0.001-0.10; S: 0.010% or less; Sol. Al: 0.01-0.10; N: 0.001-0.010; Cr: 0.2-0.6; B: 0.0010-0.0060; Sb: 0.001-0.10; Ti: 0.005-0.05; Nb: 0.003-0.08; and the remainder Fe and other inevitable impurities; and which has 0.28 or less of Ceq shown in the following formula 1, satisfies the conditions of the following formula 2, and includes bainite whose micro-structure is 30-40 area%, the remainder ferrite and martensite; and the method for manufacturing the ultra-high strength cold rolled steel sheet.[Formula 1] Ceq = C + Mn/20 + Si/30 + 2P + 4S [Formula 2] 60C - 0.2Si - 0.15Mn + 2.2Si×Cr - 0.7Cr <= 5.6 The present invention provides the ultra-high strength cold rolled steel sheet securing 980 Mpa of strength, which is an ultra-high level, and 12% of strain rate while having excellent weldability and bendability, and a method for manufacturing the ultra-high strength cold rolled steel sheet. [Reference numerals] (AA,CC,EE) Yield strength (MPa); (BB,DD,FF) Annealing temperature (°C)

Description

ULTRA HIGH STRENGTH COLD ROLLED STEEL SHEET HAVING EXCELLENT WELDABILITY AND BENDABILITY AND METHOD FOR MANUFACTURINF THE SAME}

The present invention relates to a super high strength cold rolled steel sheet excellent in weldability and bending workability and a method of manufacturing the same. More particularly, the present invention relates to a super high strength cold rolled steel sheet having a tensile strength of 980 MPa or more and a method of manufacturing the same.

Recently, fuel economy regulations have been strengthened as a task for the preservation of the global environment. As one of the countermeasures, the weight of the automobile material is reduced by increasing the strength of the steel sheet. Generally, high-strength automobile materials can be classified into precipitation hardened steel, hardened hardened steel, solidified hardened steel, and transformed tempered steel. Dual-perforated reinforced steels include Dual Phase Steel, Complex Phase Steel and Transformation Induced Plasticity steels. These transformation-strengthened steels are also referred to as Advance High Strength Steel (AHSS). DP steel is a steel in which hard martensite is dispersed finely homogeneously in soft ferrite to ensure high strength. CP steels include ferrite, martensite, bainite, two or three phases, and steels containing precipitation hardening elements such as Ti and Nb for strength improvement. TRIP steel is a type of steel that produces martensite transformation and secures high strength and high ductility when finely homogeneously dispersed austenite is processed at room temperature.

Recently, steel sheets for automobiles are required to have higher strength to improve fuel economy and durability. In view of safety of collision and protection of passengers, ultra high strength steel plates with a tensile strength of 980 MPa or more are increasingly used as car body structures and reinforcements. However, the increase in the strength of the steel sheet causes deterioration in the formability and weldability of the steel sheet. To this demand, metamorphic steel sheets such as abnormal tissue steel, TRIP steel, or composite tissue steel have been developed. For example, Patent Document 1 proposes a method of manufacturing a steel sheet excellent in moldability by controlling a chemical composition and a retained amount of austenite in a steel sheet. Patent Document 2 discloses a steel sheet having good press formability by controlling a chemical composition and a microstructure of a steel sheet A method of manufacturing a high strength steel sheet is proposed. Further, in Patent Document 3, a steel sheet excellent in workability and especially local stretching including 5% or more of retained austenite has been proposed. However, most of these inventions have been developed in order to improve the ductility, and the bending workability and weldability, which are important measures in actual part machining, have not been sufficiently considered.

Structural or reinforcing material protects passengers by absorbing collision energy at the time of collision, and if the strength of the spot welded portion is insufficient due to low weldability of such structural or reinforcing materials, collisional collision energy can not be obtained due to collision. In addition, since the extreme high strength steel material is mainly processed by bending like a side sill, the extruded steel can not be used as a part if its bendability deteriorates. The bending workability means the ratio of the minimum bending radius to the unit thickness (R / t), where the minimum bending radius ratio (R) means the minimum radius at which cracks do not occur in the outer periphery of the plate after the bending test. The requirements for bending workability are somewhat different for each automobile. However, according to Japanese Toyota Automobile, which requires the most stringent standards, it is required to satisfy the condition of R / t≤1 on the basis of a cold rolled steel sheet with a tensile strength of 980 MPa. In order to improve the bending workability, the constitution and the ratio of the transformation phase existing in the steel should be appropriately controlled. As shown in Fig. 1, a soft phase such as ferrite (F) and a soft phase such as bainite (B) or martensite It is known that the lower the strength ratio of the hard phase, the better the bending workability which can be represented by the hole expandability. For this purpose, bainite or tempered martensite has to be produced instead of martensite. However, since these transformation phases have a problem in that the yield strength rapidly increases and the elongation rate is significantly lowered as shown in Fig. 2, It is more important than that to secure properly.

On the other hand, when an ultra-high-strength steel sheet having a tensile strength of 980 MPa or more is produced in an actual process, the yield strength is also very high, so that the cold rolling property is greatly reduced due to the high strength of the hot- There is a problem that the operability is significantly lowered. In addition, since the transformation phases present in steels change very sensitively to the annealing temperature, the types and composition ratio of the transformation phases are varied by a slight change in the annealing temperature, and the yield strength is remarkably changed and the elongation is lowered. It is necessary to develop a new product capable of securing a stable material in the range. However, the prior art described above or the known technology such as Patent Document 4 have not sufficiently examined it.

Japanese Patent Application Laid-Open No. 6-145892 Japanese Patent Publication No. 2704350 Japanese Patent Publication No. 3317303 Japanese Patent Application Laid-Open No. 2005-105367

The present invention is to provide a cold-rolled steel sheet having excellent mechanical properties such as strength, elongation, weldability and bending workability by suitably controlling the type and fraction of the constituent system and microstructure, and subjecting it to annealing followed by slow- will be.

In one embodiment of the present invention, there is provided a steel sheet comprising, by weight%, 0.08 to 0.105% of C, 0.05 to 0.6% of Si, 2.5 to 2.9% of Mn, 0.001 to 0.10% of P, 0.001 to 0.10% of N, 0.001 to 0.010% of N, 0.2 to 0.6% of Cr, 0.0010 to 0.0060% of B, 0.001 to 0.10% of Sb, 0.005 to 0.05% of Ti, 0.003 to 0.08% of Nb, A cold-rolled steel sheet containing inevitable impurities and having a Ceq of 0.28 or less and satisfying the condition of the following formula 2 and having a microstructure of 30 to 40% by area of bainite and residual ferrite and martensite do.

Ceq = C + Mn / 20 + Si / 30 + 2P + 4S

[Equation 2] 60C - 0.2Si - 0.15Mn + 2.2Si x Cr - 0.7Cr? 5.6

Another embodiment of the present invention is a ferritic stainless steel comprising, by weight%, 0.08 to 0.105% of C, 0.05 to 0.6% of Si, 2.5 to 2.9% of Mn, 0.001 to 0.10% of P, 0.001 to 0.10% of N, 0.001 to 0.010% of N, 0.2 to 0.6% of Cr, 0.0010 to 0.0060% of B, 0.001 to 0.10% of Sb, 0.005 to 0.05% of Ti, 0.003 to 0.08% of Nb, Preparing a hot-rolled steel sheet containing inevitable impurities and having a Ceq of 0.28 or less represented by the following formula 1 and satisfying the condition of the following formula 2; Cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; Continuously annealing the cold-rolled steel sheet at 750 to 820 占 폚; Cooling the continuously annealed cold rolled steel sheet to a temperature of 650 to 700 ° C at a rate of 1 to 10 ° C / s; And secondarily cooling the primary-cooled cold-rolled steel sheet to a temperature of 400 to 500 ° C at a rate of 5 to 20 ° C / s, followed by overcoating.

Ceq = C + Mn / 20 + Si / 30 + 2P + 4S

[Equation 2] 60C - 0.2Si - 0.15Mn + 2.2Si x Cr - 0.7Cr? 5.6

According to the present invention, it is possible to provide a cold-rolled steel sheet having an ultra-high strength of 980 MPa or more and an elongation of 12% or more, and having excellent weldability and bending workability, and a method of manufacturing the same.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the relationship between strength ratios of soft and hard phases and hole expandability.
2 is a graph showing the relationship between elongation and hole expandability depending on the type of microstructure.
Fig. 3 is a graph showing the relationship between the yield strength deviation and Equation 2. Fig.
Fig. 4 is a graph showing a change in material according to a change in annealing temperature of an ideal structure steel. Fig. 4 (a) shows a change in yield strength, and Fig. 4 (b) shows a change in tensile strength.
Fig. 5 is a graph showing a change in material according to an annealing temperature change of steel according to an embodiment of the present invention. Fig. 5 (a) is a graph showing changes in yield strength, .

The present invention provides an ultra-high strength cold rolled steel sheet excellent in weldability and bending workability, comprising, as one embodiment, 0.08 to 0.105% of C, 0.05 to 0.6% of Si, 2.5 to 2.9% of Mn, 0.001 to 0.10% of S, 0.010% or less of S, 0.01 to 0.10% of Sol. Al, 0.001 to 0.010% of N, 0.2 to 0.6% of Cr, 0.0010 to 0.0060% of B, 0.001 to 0.10% of Sb, 0.005 to 0.05%, Nb: 0.003 to 0.08%, the balance Fe and other unavoidable impurities, Ceq expressed by the following formula 1 is 0.28 or less, the condition of the following formula 2 is satisfied, % Bainite, residual ferrite and martensite.

Ceq = C + Mn / 20 + Si / 30 + 2P + 4S

[Equation 2] 60C - 0.2Si - 0.15Mn + 2.2Si x Cr - 0.7Cr? 5.6

Hereinafter, the present invention will be described. First, the alloy composition and the composition range of the present invention will be described.

C: 0.08 to 0.105 wt%

Carbonaceous carbon (C) is a very important element added to strengthen the metamorphosis. C promotes high strength and promotes the formation of martensite in the composite structure steel. As the carbon content increases, the amount of martensite in the steel increases. However, when the content of carbon is excessively increased, it is difficult to ensure Ceq value of 0.28 or less and it is difficult to secure weldability, and the hole expandability is deteriorated. In order to solve this problem, it is preferable that the content of carbon is 0.105% or less. On the other hand, when the carbon content is less than 0.08%, the Ceq value can be in the range of 0.28 or less, but it is very difficult to secure the desired strength. Therefore, the content of C is preferably in the range of 0.08 to 0.105%.

 Si: 0.05-0.6 wt%

Silicon (Si) in steel accelerates ferrite transformation and increases the content of carbon in untransformed austenite to facilitate formation of composite structure of ferrite and martensite, and also induces solid solution strengthening effect of Si itself. Although the Si is a very useful element for improving the strength and securing the material, it may not only cause the surface scale defects of the product but also deteriorate the chemical treatment. In the present invention, the range of 0.05 to 0.6% is controlled so as not to deteriorate the weldability while securing a certain amount of ferrite and martensite. When the content of Si is less than 0.05%, sufficient ferrite is not ensured and ductility is reduced. When the content of Si exceeds 0.6%, the elongation is increased, but the strength and the spot weldability deteriorate.

Mn: 2.5 to 2.9 wt%

Manganese (Mn) in the steel fine-grains the steel without damaging the ductility and precipitates sulfur into the MnS completely in the steel to prevent hot brittleness due to the formation of FeS and strengthens the steel. At the same time, The cooling rate is lowered and martensite can be formed more easily. If the content of Mn is less than 2.5%, it is difficult to secure the desired strength in the present invention. On the other hand, when the content of Mn exceeds 2.9%, there is a high possibility of problems such as weldability and hot rolling property. To 2.9%.

P: 0.001 to 0.10 wt%

P (P) is a substitutional alloy element having the largest solid solution strengthening effect and improves the in-plane anisotropy and improves the strength. If the content is less than 0.001%, the above-mentioned effect can not be ensured and the manufacturing cost is raised. On the other hand, if the content exceeds 0.10%, the press formability is deteriorated and steel brittleness may occur, Is preferably in the range of 0.001 to 0.10%.

S: not more than 0.010% by weight

Sulfur (S) in steel is an impurity element in steel and is an element that hinders ductility and weldability of a steel sheet. If the content exceeds 0.01%, it is highly likely to deteriorate the ductility and weldability of the steel sheet, so that the content of S is preferably limited to 0.01% or less.

Sol.Al: 0.01 to 0.10 wt%

Aluminum (Al) for steel is an effective component for deoxidizing the steel in combination with oxygen in steel and for distributing carbon in ferrite to austenite like Si to improve the hardenability of martensite. If the content of Sol.Al is less than 0.01%, the above effect can not be ensured. On the other hand, when the content exceeds 0.1%, the effect is saturated and the production cost is increased. Therefore, the content of soluble Al is 0.01 to 0.1% Range.

N: 0.001 to 0.010 wt%

Nitrogen (N) in the steel is effective to stabilize austenite. When it exceeds 0.01%, the stability of austenite is greatly increased, and the formation of bainite of 30 to 40% The content of N is preferably 0.01% or less. However, considering the amount of N inevitably contained in the process and the ease of control thereof, it is difficult to control the amount of N to less than 0.001%.

Cr: 0.2 to 0.6 wt%

Chromium (Cr) in steel is a component added to improve the hardenability of steel and secure high strength. In the present invention, it is an element that plays a very important role as a bainite formation promoting element. If the content of Cr is less than 0.2%, it is difficult to ensure the above effect. If it exceeds 0.6%, the elongation due to excessive strength increase is deteriorated as well as economically. Therefore, the content of Cr is in the range of 0.2 to 0.6% .

B: 0.0010 to 0.0060 wt%

Boron (B) in steel is a component that delays transformation of austenite into pearlite during cooling during annealing, and is an element that inhibits ferrite formation and promotes the formation of bainite. However, when the content of B is less than 0.0010%, it is difficult to obtain the above effect. When the content of B exceeds 0.0060%, excess B may be concentrated on the surface to deteriorate the plating adhesion, %. &Lt; / RTI &gt;

Sb: 0.001 to 0.10 wt%

Antimony (Sb) is a component to be added in order to ensure excellent dent resistance in the present invention. Wherein Sb is MnO, SiO _2, Al ₂ O ₃ sikimyeo by suppressing the surface enrichment of the oxide, such as reducing the surface defects due to dents, to suppress the coarsening of surface agglomerates according to the temperature rise and the hot rolling step change very effective . If the content of Sb is less than 0.001%, it is difficult to secure the above effect. If the content exceeds Sb, the effect is saturated and the production cost and workability may be deteriorated. It is preferably in the range of 0.005 to 0.1%.

0.005 to 0.05% by weight of Ti, 0.003 to 0.08% by weight of Nb,

Titanium (Ti) and niobium (Nb) in steel are effective elements for increasing the strength and fineness of the steel sheet. If the content of Ti is less than 0.005% or the content of Nb is less than 0.003%, it is difficult to ensure the above effect. If the content of Ti exceeds 0.05% or the content of Nb exceeds 0.08% The ductility can be largely lowered due to the increase of the amount of the precipitate and the excessive precipitation. Therefore, it is preferable that the Ti content is 0.005 to 0.05% by weight and the Nb content is 0.003 to 0.08% by weight.

The steel sheet of the present invention preferably has Ceq in the range of 0.28 or less in addition to the component system described above.

Ceq = C + Mn / 20 + Si / 30 + 2P + 4S

Equation 1 is a component relational expression for securing the weldability of a steel sheet. In the present invention, by controlling the Ceq value to 0.28 or less, excellent spot weldability can be ensured and excellent mechanical properties can be imparted to the welded portion. If the Ceq exceeds 0.28, the weldability may deteriorate and the physical properties of the welded portion may deteriorate.

The steel sheet of the present invention preferably satisfies the condition of the following formula (2) in addition to the above formula (1).

[Equation 2] 60C - 0.2Si - 0.15Mn + 2.2Si x Cr - 0.7Cr? 5.6

According to the studies of the present inventors, it has been found that the proper addition of the alloying constituents described in the above-mentioned formula 2 is very important in order to reduce the yield strength variation with the change of the annealing temperature. Generally, the annealing process is performed after a specific temperature is set within a certain range. However, even if the annealing is performed after the temperature is fixed in this way, it is difficult to keep the temperature constant for the entire annealing furnace in the manufacturing process. In other words, a temperature change or a deviation is inevitably generated in the annealing step, and it becomes considerably difficult to obtain the kind and fraction of the target microstructure due to the temperature change. As a result, the mechanical properties such as the yield strength vary . The above formula 2 is intended to stably ensure the fraction of the bainite to be obtained even when the temperature changes during the annealing step to reduce the material deviation such as the yield strength. Fig. 3 is a graph showing the relationship between the yield strength deviation and Equation 2. Fig. As shown in Fig. 3, in order to secure the deviation of the yield strength to be a stable range of 80 MPa or less, the value of the formula 2 is required to be in the range of 5.6 or less, and if it exceeds 5.6, .

On the other hand, the steel sheet of the present invention can obtain excellent mechanical properties when the alloy composition and the composition range described above are satisfied, but may further include Mo for more preferable effects.

Mo: 0.02% by weight or less

Molybdenum (Mo) in steel is a very advantageous element for improving the hardenability of steel and securing high strength like Cr. In addition, Mo-based fine carbides are generated in the steel to improve the strength of the ferrite base structure. However, Mo is an alloy element which is about 10 times more expensive than Cr, which leads to an excessive increase in manufacturing cost. Therefore, it is preferable to reduce the addition amount as much as possible in order to reduce the production cost when the Mo is added. That is, it is preferable that the Mo has a range of 0.02% or less which can provide an excellent effect while preventing an excessive increase in manufacturing cost.

The cold-rolled steel sheet according to one embodiment of the present invention preferably has a microstructure of 30 to 40% by area of bainite and a main structure of ferrite and martensite. When the bainite content is less than 30% by area, the bending workability is lowered due to the increase of martensite, so that the target R / t of the present invention can not have a range of 1 or less. When the bainite content exceeds 40% The bending workability is good but the ductility is lowered and the yield strength is remarkably increased due to the formation of excessive bainite.

On the other hand, the steel sheet of the present invention can contain ferrite and martensite as a main structure, thereby securing excellent mechanical properties. The ferrite serves to improve the ductility of the steel, and the martensite plays a role of increasing the strength, so that the tensile strength to be obtained by the present invention can be ensured. In order to increase the strength and improve the ductility, it is preferable that the martensite is finely and uniformly distributed in the ferrite. The fraction of the ferrite and martensite is not particularly limited, but the ferrite may have a range of 30 to 40 area%, and the martensite may have a range of 20 to 30 area%. If the ferrite content is less than 30%, the desired ductility of the present invention can not be expected due to the lack of elongation. If the ferrite content is more than 40%, the elongation is increased but the strength is lowered due to lack of martensite and bainite It can cause problems. If the martensite content is less than 20%, it is difficult to expect an increase in strength due to the deficiency of the transformation phase responsible for strengthening. If the martensite content exceeds 30%, the excessive strength increase and the difference in strength between the ferrite and martensite phases becomes large, It can cause problems.

The cold-rolled steel sheet of the present invention as described above can have excellent mechanical properties such as yield strength of 700 MPa or less, tensile strength of 980 MPa or more, elongation of 12% or more, and bending workability (R / t) of 0.1. As described above, since the steel sheet of the present invention has high strength, excellent weldability and bending workability, it can be preferably applied to automobiles requiring excellent strength and moldability. Further, the steel sheet of the present invention may further comprise a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer for use in automobiles and the like. On the other hand, the bending workability means a value obtained by dividing the minimum radius curvature R at which cracks do not occur, by the steel sheet thickness t.

Hereinafter, the production method of the present invention will be described.

One of the most important features of the present invention is to secure the stability of the material. Generally, in order to produce a two-phase structure steel, an appropriate amount of ferrite and austenite are secured by annealing-cracking treatment in the two-phase region of Ar1 to Ar3, and then austenite is transformed into martensite through quenching, There is a method of securing a microstructure having a phase structure. The characteristic of these steels is to secure ductility by soft ferrite and to secure the desired level of strength through martensite. However, since the difference in strength between the two phases is very large, cracks can easily occur at the boundary of the phase when deformation is applied. These cracks are a main factor for deteriorating hole expandability or bending workability. To solve this problem, techniques for reducing the amount of martensite and generating bainite or tempered martensite to reduce the difference in phase hardness between two-phase structure steels have also been developed. More specifically, after increasing the temperature of the annealing to more than Ar3 to secure austenite of 100%, bainite and martensite are secured as a small amount of ferrite and main phase through cooling, or quenched in a bimetallic furnace and quenched to obtain ferrite + After securing the site, the carbide is precipitated in the martensite through tempering (tempered martensite) to reduce the strength difference between the phases. However, the above-mentioned techniques have a problem that the yield ratio (YR) increases due to an excessive increase in the yield strength relative to the tensile strength, and the elongation is remarkably lowered. In addition, there is a disadvantage that weldability is deteriorated due to excessive addition of alloying elements.

In order to secure weldability, bending workability, strength and ductility at the same time, it is necessary to control rigid alloy composition and to establish operating conditions. However, even if these conditions are established, there remains a significant problem to be solved. The change of the material, especially the yield strength, is very severe with the change of the annealing temperature. More specifically, even if annealing is performed in a two-phase region, if the annealing temperature is changed in the two-phase region, the content of austenite is changed. As a result, the carbon content in the austenite is changed. nose) is moved. In other words, when the carbon concentration in the austenite decreases due to an increase in the annealing temperature, the martensite transformation is relatively delayed and the bainite transformation is promoted to increase the amount of bainite. However, when the annealing temperature is low and the carbon concentration in the austenite is increased, the bainite transformation nose shifts to the right side, that is, the retardation increases the martensite generation relatively easily. Thus, even in the same two-phase zone, the production of bainite varies depending on the degree of carbon enrichment in the austenite due to the change in the annealing temperature. Due to this change, not only the material but also the deviation of the yield strength becomes very large, . Fig. 4 is a graph showing a change in material according to a change in annealing temperature of an ideal structure steel. Fig. 4 (a) shows a change in yield strength, and Fig. 4 (b) shows a change in tensile strength. As shown in Fig. 4, the strength varies greatly with the change of the annealing temperature. Particularly, the section of the annealing temperature for securing a tensile strength of 980 MPa or more while satisfying the yield strength of 700 MPa or less is 770 to 785 캜, It can be seen that an adequate material can be secured only in a very narrow range. However, as mentioned above, it is considerably difficult to maintain the annealing temperature at a constant level. In order to prevent the material from being varied in accordance with the temperature change, the deviation of the material can be suppressed as much as possible regardless of the annealing temperature. The development of technology that is required Generally, the range of the annealing temperature required for the minimum operation is ± 10 ° C, and in this respect, it is necessary to secure an annealing temperature range of at least ± 15 ° C or more.

The present invention solves the above-mentioned problems, and improves ductility, weldability, bending workability and strength. In particular, it is an object of the present invention to provide a cold-rolled steel sheet having a material exhibiting a variation in yield strength of 80 MPa or less Preparing, as an embodiment, a hot-rolled steel sheet satisfying the above-described alloy component and composition range; Cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; Continuously annealing the cold-rolled steel sheet at 750 to 820 占 폚; Cooling the continuously annealed cold rolled steel sheet to a temperature of 650 to 700 ° C at a rate of 1 to 10 ° C / s; And secondarily cooling the primary-cooled cold-rolled steel sheet to a temperature of 400 to 500 ° C at a rate of 5 to 20 ° C / s, followed by overcoating.

First, a hot-rolled steel sheet having the above composition is prepared. The hot-rolled steel sheet can be produced by a method commonly used in the art. That is, it can be obtained by reheating a steel slab having the above-described alloy composition, followed by hot finishing rolling and winding. However, in order to obtain a more preferable effect, it is preferable that the hot-rolled steel sheet is subjected to the hot-rolling at 800 to 950 ° C, and the winding is performed at 500 to 750 ° C.

If the finish hot rolling temperature is less than 800 ° C, there is a high possibility that the hot deformation resistance will increase sharply and the top, tail and edge of the hot-rolled coil become single-phase regions, &Lt; / RTI &gt; If the temperature is higher than 950 ° C, an excessively large oxidation scale may occur, and the microstructure of the steel sheet may be coarsened. Therefore, the finish hot rolling temperature is preferably in the range of 800 to 950 ° C.

If the coiling temperature is less than 500 占 폚, excessive martensite or bainite is generated to cause an excessive increase in the strength of the hot-rolled steel sheet, which may cause problems such as a shape defect due to a load during cold rolling. When the temperature is higher than 750 ° C, the surface hardening becomes severe due to elements that lower the wettability of hot dip galvanizing such as Si, Mn, and B, and it may be difficult to ensure excellent plating performance. Therefore, it is preferable that the coiling temperature has a temperature range of 500 to 750 占 폚.

The hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio in the cold rolling is preferably in the range of 40 to 70%. If the cold rolling reduction rate is less than 40%, the recrystallization driving force is weakened and there is a problem that a good recrystallized grain is obtained, and the shape correction is very difficult. If it exceeds 70%, there is a high possibility that cracks occur at the edge of the steel sheet, and the rolling load may increase sharply. Therefore, it is preferable that the cold rolling is performed at a reduction ratio of 40 to 70%. On the other hand, before the cold rolling, pickling may be performed to remove scales, impurities, etc. adhering to the surface.

Thereafter, the cold-rolled steel sheet is continuously annealed at 750 to 820 캜. When the continuous annealing temperature is less than 750 ° C, the risk of non-recrystallized grains is increased and it is difficult to form sufficient austenite, so that it is difficult to secure a desired strength in the present invention. If the temperature exceeds 820 DEG C, the content of bainite increases sharply due to formation of excessive austenite, resulting in an excessive increase in yield strength and deterioration of ductility. Therefore, the continuous annealing temperature is preferably in the range of 750 to 820 캜.

On the other hand, it is preferable to carry out the cracking treatment for 30 seconds or more after the continuous annealing. This is to ensure a sufficient fraction of ferrite at the annealing temperature suggested by the present invention in addition to recrystallization and grain growth of the cold rolled structure. That is, when the cracking time is shorter than 30 seconds, the steel can not be completely recrystallized and the ductility may deteriorate in the final structure. In addition, there is a possibility that the generation of the transformation phase in the heat- high. On the other hand, since the above-mentioned cracking treatment can be expected to have a desirable effect when carried out for as long as possible, the upper limit of the cracking treatment time is not particularly limited in the present invention. However, those skilled in the art can appropriately control the cracking time in consideration of the productivity.

Then, the cold-rolled steel sheet is firstly cooled to 650 to 700 ° C at a rate of 1 to 10 ° C / s. The primary cooling step is to increase the ductility and strength of the steel sheet by securing the equilibrium carbon concentration of the ferrite and the austenite. When the primary cooling termination temperature is lower than 650 캜 or higher than 700 캜, it is difficult to secure the ductility and strength desired in the present invention. Therefore, it is preferable that the primary cooling end temperature is in the range of 650 to 700 ° C.

After the primary cooling, the cooled cold-rolled steel sheet is secondarily cooled to a temperature of 400 to 500 ° C at a rate of 5 to 20 ° C / s and subjected to an overbasing treatment in a bainite section. The secondary cooling is one of the control factors that is important in the present invention. The secondary cooling termination temperature is a very important condition for ensuring both ductility and bending workability. When the cooling termination temperature is lower than 400 占 폚, it is difficult to obtain a sufficient bainite fraction because of the short staying time in the bainite region during the over- , And when it exceeds 500 ° C, the time for staying in the bainite region during overexposure treatment becomes very large, and the fraction of bainite becomes excessive, thereby increasing the yield strength and deteriorating ductility. When the secondary cooling rate is less than 5 ° C / s, ferrite transformation occurs preferentially before and after the bainite transformation due to the slow cooling rate, which results in a disadvantage in that an adequate amount of bainite can not be obtained. ° C / s, martensite may preferentially be formed before the bainite transformation due to excessive cooling rate, resulting in a decrease in ductility due to an increase in the amount of martensite.

The overexposure treatment is preferably performed at 300 to 400 캜. When the overheating treatment stop temperature is less than 300 ° C, martensite is produced first. If the overheating treatment temperature is more than 400 ° C, the ferrite production amount is increased and the strength is lowered.

The present invention can easily obtain a desired amount of bainite through annealing and annealing after annealing. Thus, a superior level of bending workability can be secured. Of course, even if the above-mentioned heat treatment conditions are satisfied, a desired material can not be secured if the above-mentioned alloy composition, particularly the condition of the formula (2) is not satisfied.

On the other hand, skin pass rolling can be additionally performed at a reduction rate of 0.1 to 0.5% after the overexposure treatment. The skin pass rolling serves to control the shape of the cold-rolled steel sheet and raise the yield strength to a certain level. When the skeleton of the skeleton of the present invention is subjected to skin pass, it is possible to increase the yield strength of at least 50 MPa or more without increasing the tensile strength. If the skin pass reduction rate is less than 0.1%, the shape control of the ultra-high strength steel as in the present invention may be very difficult. If it exceeds 0.5%, the yield strength may exceed 700 MPa due to an excessive increase in yield strength In addition to this, the operation ability can be greatly unstable.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are only illustrative of the present invention in more detail and do not limit the scope of the present invention.

(Example)

A steel slab having an alloy composition as shown in Table 1 below was reheated in a furnace at 1200 DEG C for one hour, followed by hot rolling, and then wound. At this time, the hot finish rolling temperature was 890 占 폚, and the coiling temperature was set at 650 占 폚. Thereafter, pickling was carried out using a hot-rolled steel sheet, and cold rolling was carried out at a cold rolling reduction of 50%. The cold-rolled steel sheet was annealed and cooled under the conditions shown in Table 2 below, and skin pass was performed at a final reduction ratio of 0.3%. Then, JIS No. 5 tensile test specimens were produced from the thus-prepared cold-rolled steel sheets to measure mechanical properties such as yield strength, tensile strength, elongation and bending workability, and microstructure, and the results are shown in Table 3 below. The bending workability was measured by bending test under the condition that R / t, which is the value obtained by dividing the minimum radius curvature (R) by the steel sheet thickness (t), was 1, And x for the material. In addition, in order to measure the yield strength deviation, the same manufacturing conditions as those described in Table 2 below were applied to all the steel types, only the annealing treatment was performed in various temperature ranges, and then the value obtained by subtracting the minimum value from the maximum value of the yield strength Yield strength deviations.

division Chemical composition (% by weight) C Si Mn P S Sol.Al Cr Ti Nb N B Sb Equation 1 Equation 2 Inventory 1 0.09 0.4 2.7 0.01 0.003 0.035 0.4 0.02 0.035 0.005 0.0025 0.03 0.270 4.99 Inventive Example 2 0.09 0.4 2.7 0.009 0.002 0.05 0.4 0.021 0.035 0.005 0.0025 0.02 0.264 4.99 Inventory 3 0.09 0.4 2.7 0.01 0.005 0.035 0.5 0.022 0.035 0.003 0.0025 0.03 0.278 5.01 Honorable 4 0.1 0.3 2.7 0.009 0.004 0.06 0.3 0.05 0.035 0.004 0.0025 0.04 0.279 5.52 Inventory 5 0.1 0.1 2.7 0.01 0.003 0.07 0.3 0.03 0.035 0.0056 0.0025 0.02 0.270 5.43 Inventory 6 0.1 0.1 2.7 0.01 0.003 0.05 0.5 0.02 0.035 0.0045 0.0025 0.03 0.270 5.34 Honorable 7 0.1 0.2 2.7 0.01 0.003 0.04 0.4 0.025 0.035 0.0047 0.0025 0.02 0.274 5.45 Honors 8 0.1 0.1 2.7 0.011 0.005 0.055 0.4 0.025 0.04 0.007 0.0025 0.04 0.280 5.38 Proposition 9 0.08 0.4 2.7 0.011 0.004 0.045 0.4 0.025 0.045 0.006 0.0025 0.03 0.266 4.39 Inventory 10 0.08 0.3 2.9 0.012 0.005 0.06 0.4 0.025 0.03 0.0065 0.0025 0.03 0.279 4.29 Comparative Example 1 0.09 0.6 3.0 0.01 0.003 0.05 0.4 0.02 0.04 0.0041 0.0005 0.04 0.292 5.08 Comparative Example 2 0.09 0.6 3.1 0.011 0.004 0.05 0.4 0.02 0.045 0.0035 0.0005 0.02 0.303 5.06 Comparative Example 3 0.09 0.6 3.1 0.01 0.005 0.04 0.4 0.03 0.04 0.0055 0.0005 0.05 0.305 5.06 Comparative Example 4 0.09 0.6 3.2 0.012 0.003 0.035 0.4 0.02 0.03 0.006 0.0005 0.03 0.306 5.05 Comparative Example 5 0.09 0.8 2.8 0.009 0.005 0.035 0.4 0.02 0.035 0.007 0.0025 0.04 0.295 5.24 Comparative Example 6 0.11 0.6 2.5 0.01 0.006 0.04 0.4 0.02 0.035 0.005 0.0025 0.02 0.299 6.35 Comparative Example 7 0.13 0.6 2.2 0.011 0.005 0.03 0.4 0.02 0.035 0.006 0.0025 0.03 0.302 7.60 Comparative Example 8 0.09 0.1 3.0 0.011 0.006 0.05 0.4 0.025 0.035 0.0065 0.0025 0.05 0.289 4.74 Comparative Example 9 0.15 0.1 2.7 0.008 0.002 0.04 0.4 0.02 0.035 0.004 0.0025 0.04 0.312 8.38 Comparative Example 10 0.1 0.4 3.5 0.011 0.003 0.06 0.4 0.03 0.03 0.005 0.0025 0.03 0.322 5.47 Formula 1: C + Mn / 20 + Si / 30 + 2P + 4S
Equation 2: 60C - 0.2Si - 0.15Mn + 2.2Si x Cr - 0.7Cr

division Annealing temperature
(℃) Crack treatment
time
(second) Primary
Cooling rate
(° C / s) Primary
Cooling stop temperature
(℃) Secondary
Cooling rate
(° C / s) Secondary
Cooling stop temperature
(℃) Inventory 1 790 60 3 650 15 460 Inventive Example 2 800 60 3 650 15 450 Inventory 3 790 60 3 650 15 460 Honorable 4 780 60 3 650 15 450 Inventory 5 790 60 3 650 15 460 Inventory 6 780 60 3 650 15 450 Honorable 7 790 60 3 650 15 460 Honors 8 800 60 3 650 15 450 Proposition 9 790 60 3 650 15 460 Inventory 10 780 60 3 650 15 450 Comparative Example 1 800 60 3 650 15 460 Comparative Example 2 780 60 3 650 15 250 Comparative Example 3 790 60 3 650 15 300 Comparative Example 4 780 60 3 650 15 450 Comparative Example 5 800 60 3 650 15 460 Comparative Example 6 780 60 3 650 15 450 Comparative Example 7 790 60 3 650 15 460 Comparative Example 8 780 60 3 650 15 450 Comparative Example 9 790 60 3 650 15 460 Comparative Example 10 780 60 3 650 15 450

division Yield strength
(MPa) The tensile strength
(MPa)
Elongation
(%) Yield ratio Yield strength
Deviation
(MPa) flex
Processability Bay
Night
(area%) Inventory 1 654.6 1023.4 14.4 0.64 42.4 ○ 31 Inventive Example 2 696.8 1005.1 13.6 0.69 76.4 ○ 35 Inventory 3 665.8 1090.7 13.3 0.61 63 ○ 36 Honorable 4 693.37 1029.59 12.2 0.67 68.52 ○ 37 Inventory 5 643.9 1024.4 12.7 0.63 77.8 ○ 33 Inventory 6 629.4 1008.3 12.5 0.62 73.8 ○ 34 Honorable 7 654 1040.7 13.0 0.63 36.2 ○ 33 Honors 8 599.4 1060 14.6 0.57 22.8 ○ 31 Proposition 9 618.4 1014.5 16.4 0.61 47.6 ○ 31 Inventory 10 623.3 1025 13.8 0.61 75.8 ○ 33 Comparative Example 1 620.9 1019.1 13.8 0.61 99.4 × 35 Comparative Example 2 742.3 1029.2 8.6 0.72 271.0 × 15 Comparative Example 3 741.3 1003.5 7.9 0.74 223.4 × 21 Comparative Example 4 677.4 1052.6 13.4 0.64 248.8 × 39 Comparative Example 5 705.7 1109.4 12.6 0.64 126.2 × 45 Comparative Example 6 800.5 1095.2 10.5 0.73 171.2 × 81 Comparative Example 7 750.9 1165.2 10.5 0.64 181.2 × 72 Comparative Example 8 962.8 1256.9 5.6 0.77 100.4 × 80 Comparative Example 9 821.5 1120.3 8.6 0.73 240.6 × 65 Comparative Example 10 750.1 1092.2 10.5 0.69 211.8 × 42

As shown in Tables 1 to 3, Inventive Examples 1 to 10 satisfying the alloy composition and manufacturing conditions proposed by the present invention satisfied the yield strength of 700 MPa or less, the tensile strength of 980 MPa or more, and the elongation of 12% or more , And the value of the expression (1) representing the spot weldability at the same time satisfies 0.28 or less, indicating excellent weldability. The inventive examples 1 to 10 all have a bainite fraction in the range of 30 to 40% by area, so that R / t, which is an index of bending workability, satisfies the range of 1.0 or less and ensures excellent bending workability, 2 is 5.6 or less, it can be seen that the deviation of the yield strength is within the range of 80 MPa and has a very uniform material characteristic even in a wide annealing range such as the annealing temperature ± 20 ° C. These characteristics are the levels that can significantly reduce other defects in material variation during machining with automotive parts.

FIG. 5 is a graph showing a change in material according to the annealing temperature change in Inventive Example 1. FIG. 5 (a) shows a change in yield strength, FIG. 5 (b) shows a change in tensile strength and FIG. As shown in Fig. 5, it can be confirmed that the yield strength and the tensile strength of the inventive example 1 change very little depending on the change of the annealing temperature. Particularly, the yield strength change of about 40 MPa Able to know. And the elongation is also excellent.

On the other hand, in the case of Comparative Examples 1 to 10, which do not satisfy the alloy composition or the manufacturing conditions proposed by the present invention, most of the bainite fraction proposed by the present invention is not secured, and the weldability is deteriorated or the yield strength is increased It can be seen that the deviation of the yield strength is also excessively increased. Further, it can be seen that the elongation decreases and the bending workability does not satisfy the target level proposed by the present invention.

Claims (10)

0.001 to 0.10% of S, 0.010% or less of S, 0.01 to 0.10% of Sol.Al, 0.01 to 0.10% of N, 0.001 to 0.10% of Sn, 0.05 to 0.6% of Si, 0.001 to 0.10% of Sb, 0.005 to 0.05% of Ti, 0.003 to 0.08% of Nb, the balance of Fe and other unavoidable impurities,
Ceq expressed by the following formula 1 is 0.28 or less,
Satisfy the condition of the following formula (2)
Wherein the microstructure is composed of 30 to 40% by area of bainite, residual ferrite and martensite.
Ceq = C + Mn / 20 + Si / 30 + 2P + 4S
[Equation 2] 60C - 0.2Si - 0.15Mn + 2.2Si x Cr - 0.7Cr? 5.6 The method according to claim 1,
Wherein the cold-rolled steel sheet further contains 0.02% or less Mo. The method according to claim 1,
The cold-rolled steel sheet has a yield strength of 700 MPa or less, a tensile strength of 980 MPa or more, an elongation of 12% or more, and a bending workability (R / t) of 0.1.
(Wherein the bending workability is a value obtained by dividing the minimum radius curvature R at which cracks do not occur, by the steel sheet thickness t). 0.001 to 0.10% of S, 0.010% or less of S, 0.01 to 0.10% of Sol.Al, 0.01 to 0.10% of N, 0.001 to 0.10% of Sn, 0.05 to 0.6% of Si, 0.001 to 0.10% of Sb, 0.005 to 0.05% of Ti, 0.003 to 0.08% of Nb, the balance of Fe and other unavoidable impurities,
Ceq expressed by the following formula 1 is 0.28 or less,
Preparing a hot-rolled steel sheet satisfying a condition of Formula 2 below;
Cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet;
Continuously annealing the cold-rolled steel sheet at 750 to 820 占 폚;
Cooling the continuously annealed cold rolled steel sheet to a temperature of 650 to 700 ° C at a rate of 1 to 10 ° C / s; And
And cooling the first cooled cold rolled steel sheet to 400 to 500 ° C. at a rate of 5 to 20 ° C./s, followed by overaging treatment.
Ceq = C + Mn / 20 + Si / 30 + 2P + 4S
[Equation 2] 60C - 0.2Si - 0.15Mn + 2.2Si x Cr - 0.7Cr? 5.6 The method of claim 4,
Wherein the hot-rolled steel sheet further contains 0.02% or less Mo. The method of claim 4,
The hot-rolled steel sheet is obtained by reheating a steel slab, then hot rolling at 800 to 950 ° C, and winding at 500 to 750 ° C. The method of claim 4,
Wherein the cold rolling is performed at a reduction ratio of 40 to 70%. The method of claim 4,
And performing a cracking treatment for 30 seconds or more after the continuous annealing. The method of claim 4,
The overaging treatment is carried out to 300 ~ 400 ℃ manufacturing method of cold rolled steel sheet. The method of claim 4,
Wherein the skin pass rolling is performed at a reduction ratio of 0.1 to 0.5% after the overaging treatment.

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